The present invention is related to a detection apparatus, and more particularly to a load detection apparatus.
Nowadays, the presence of peripheral devices in a computer system is subject to an automatic load detection for automatically adapting the system configuration in order that the user can freely install various kinds of peripheral devices.
FIG. 1 is a schematic circuit diagram showing a conventional load detection apparatus electrically connected to a peripheral device 11. The conventional load detection apparatus includes a potential comparing device 12 and a load-connecting medium 13, wherein the potential comparing device 12 further includes a constant resistor 121 serially connected to a power source V.sub.cc, a first voltage-dividing resistor 123, a second voltage-dividing resistor 124, and an operational amplifier 122. The conventional detection principle is described below.
If there is no peripheral device load electrically connected to the load-connecting medium 13, the potential measured at a point X will be V.sub.cc owing to the open circuit state of the load-connecting terminal 13. As for the potential measured at a point Y, it is obtained by dividing the voltage from V.sub.cc through the first and the second voltage-dividing resistors 123 and 124. A low level signal generated at a terminal Z to represent a loadless state is obtained by applying the potentials measured at the points X and Y to the operational amplifier 122.
However, when the peripheral device load 11 is electrically connected to the load-connecting terminal 13, a load current I.sub.L flows through the constant resistor 121 and the peripheral device load 11, and the voltage at the point X is then divided through the constant resistor 121 and the peripheral device load 11. Therefore, the potential measured at the point X will be lower than that measured at the point Y and thus the operational amplifier 122 will generate a high level signal at the terminal Z to represent the presence of a load.
The conventional technique for load detection has the shortcoming that it requires the resistance of the constant resistor 121 to have a constant value within a certain range. If the resistance is too small, a load-present signal will not be generated at the point Y when the load-connecting medium 13 is electrically connected to a peripheral device having a small load. On the other hand, if the resistance is too large, a large voltage drop will be generated when the load current I.sub.L flows through the constant resistor 121 so that the output voltage provided for the peripheral device through the load-connecting terminal 13 will be insufficient to drive the peripheral device when the load-connecting terminal 13 is electrically connected to a peripheral device having a heavy load.
For example, if the peripheral device 11 can be detected to be present when the voltage drop V.sub.ax across the constant resistance 121 is 0.1 V and the load current I.sub.L is 1 A, the constant resistor 121 should be designed to have a resistance of 100 m.OMEGA.. When the load-connecting terminal 13 is electrically connected to a peripheral device having such a small load that the load current I.sub.L intensity drops to 1 mA, V.sub.ax will be reduced to 0.1 mV and the potential at the point X is still higher than that at the point Y so that the load-present signal still cannot be generated at the terminal Z. Alternatively, if the load current intensity is 1 mA, then the constant resistor 121 should be designed to have a resistance of 100 .OMEGA.. When the load-connecting medium 13 is electrically connected to a peripheral device having such a heavy load that the load current I.sub.L intensity rises to 50 mA, the voltage drop V.sub.ax will rise to the power source voltage V.sub.cc so that the load-connecting terminal 13 cannot supply a sufficient voltage to drive the peripheral device.
To sum up, the conventional load detection technique cannot be widely used for detecting various kinds of load devices, because a correct signal indicative of the presence of a peripheral device cannot be obtained once the peripheral device load falls out of a certain range.